Fix incremental-snapshot

[Dunedan]

incremental-snapshot didn't work for me without this change. Instead it would throw the following exception:

TypeError: Cannot read property 'toString' of undefined
    at Transform.stringify._transform
    (/some/path/dynamodb-replicator/s3-snapshot.js:43:33)
    at Transform._read (_stream_transform.js:167:10)
    at Transform._write (_stream_transform.js:155:12)
    at doWrite (_stream_writable.js:329:12)
    at writeOrBuffer (_stream_writable.js:315:5)
    at Transform.Writable.write (_stream_writable.js:241:11)
    at Transform.ondata (_stream_readable.js:557:20)
    at emitOne (events.js:96:13)
    at Transform.emit (events.js:191:7)
    at readableAddChunk (_stream_readable.js:178:18)

[greenu]

Please merge